Plastic brushes of the kind for which this invention has a particular advantage consist essentially of two elements: the bristles and the handle. At the present time, such brushes are produced in conventional injection molds as integral units, the molds including a multicell bristle cavity and an adjoining handle cavity.
During injection, the plastic material (typically polyethylene) fills the bristle cavity and the handle cavity. When the mold is opened, the bristles which are attached to the handle are pulled out of their cavities by the handle which is held in its cavity by undercuts or by side cores. Once the bristles have been released from these cavity cells, the brush is ejected from the handle cavity.
The cells of the bristle cavity can be formed by simple blocks, taper-drilled for the shape of the bristles, or composite blocks, consisting of a pack of interfitting blades into which bristle-shaped grooves are machined. The bristle shape is typically triangular in cross-section and tapers toward the tip. These blade packs can be fixed or relatively slidable, as described in my earlier U.S. Pat. Nos. 3,004,291 and 3,128,488.
Another method of making such bristle-molding cells utilizes cavity blocks with a multitude of round pins inserted into them. Each pin has a plurality of axially oriented grooves in its circumference, typically of triangular shape, whereby several bristles are formed by each pin.
Regardless of the bristle-cavity construction which is used, it will be readily understood that the cooling of the bristles gives rise to problems relating to the length of time required. Because the bristles of an ordinary brush are spaced very close to one another, it is virtually impossible to provide cooling channels where they are most needed, namely right next to the bristles. The only practical way to provide any cooling is to locate the channels around the cavity blocks and in the backing plate. This construction results in an inefficient and therefore slow cooling of the bristles. In fact, cooling cycles last 1/2 to 11/2 minutes, depending upon the length and thickness of the bristles.
A serious drawback with the brushes produced from polyethylene is the fact that, even with very fine bristles, they do not generate the kind of lather required for such uses as "scrubbing" by medical personnel.
A relatively new molding material, known commercially as "Kraton" (trademark), is a modified rubber that can be processed in conventional injection-molding equipment. As used hereinafter, the term "modified rubber" is intended to mean a material substantially the same as is available under the name of Kraton. This material is more expensive than polyethylene, yet the brushes produced with it make an excellent lather. Because Kraton is very "rubbery" (elastic and flexible) it is an excellent material for the bristles of the brush, but this same characteristic makes it almost impossible to mold Kraton with the previously described methods of ejection. In other words, if Kraton were used to make an integral brush in which undercuts or side cores in the handle served to pull on the handle and thus release the bristles from the cells of their cavity, the "stickiness" of the bristles against the small-bore cells would cause them to remain stuck and literally pull the handle out of its own cavity, even if it were strongly engaged by undercuts or side cores. The Kraton simply stretches and slips out uncontrollably, leaving the bristles of the brush in associated elongate mold cells.
This difficulty can be overcome by attaching a rigid backing to the base of the bristles, either by fastening or by molding, which can be used to pull the bristles out of their cells in a controlled manner.
It should be noted that the above remarks are not limited to Kraton or to brushes, but can be applied to any soft, rubbery material and for any product which is difficult to remove automatically from the cavities by conventional methods.